This invention relates to a method for use in offset printing. This invention also relates to an associated apparatus.
In offset printing, the matter to be reproduced is copied photographically onto a metal plate disposed on a cylinder of a printing press. The metal plate is then inked and rotated with the plate cylinder to make an imprint of the text or illustration on a rubber layer disposed on a respective cylinder. This cylinder is in turn rotated to print on sheets of paper which are automatically fed to the machine. An impression cylinder serves as the back-pressure cylinder which squeezes the paper sheets to the rubber-covered cylinder for transferring ink to the substrate for final reproduction. In addition, a coater apparatus is attached to a deck area of the printing press. When a job requires coating, the coater apparatus is shifted into direct contact with the rubber-covered cylinder while the plate cylinder is not operable to transfer ink images to the rubber surface.
Each cylinder of the offset printing press has a gap area for allowing clearance of a delivery gripper chain system. When a reproduction of spot coating is required, the printer has several options: he may purchase a DuPont CYREL (trademark) plate or a single, dual or multilayer printing blanket which is used in the offset process for transferring inked images, which is manufactured by Reeves International or Day International, Inc. The construction of the printing blanket can be either a compressible or a conventional design. The DuPont CYREL.TM. plate has a flexible photopolymer compound laminated in a layer to a polymeric carrier layer such as polyvinyl chloride or MYLAR.TM.. A chemical developer is used to remove non-imaging, unexposed areas from the photopolymer compound layer of the plate. There follows a light exposure finishing procedure to harden the remaining polymer. After developing, the plate has two surface levels. A lower surface area is the non-image area, which does not receive a coating during a printing process, while a raised surface area receives a coating from the coating apparatus. The differential in height is referred to as the relief area. This relief area ranges from 10 to 30 thousandths of an inch for the CYREL.TM. system.
An offset printing blanket for providing a raised surface for spot coating reproduction is also commonly used by offset printers. The blanket has an upper rubber layer removably attached to an underlying, woven fabric layer. The printing blanket is attached to the blanket cylinder so that the fabric layer is disposed inside, in contact with the cylinder, while the rubber layer is outside. If the blanket is pre-installed around its cylinder, a manual or freehand cutting of the rubber layer is required. This manual or freehand cutting technique is time consuming and limited in practice to extremely simple patterns because of the difficulty in achieving accuracy.
Some printers cut the rubber layer of a printing blanket before installing it on the blanket cylinder of an offset printing press. This process is accomplished by using a multicutting plotter, for example, manufactured by Misomex, which provides accurate cuts over a wide range of complicated shapes and designs. The rubber blanket is cut in a flat orientation and subsequently attached to the blanket cylinder. Distortion introduced by the mounting tension requires special compensating steps, in order to reproduce the coating in the exact location of the printed design once the carrier material is wrapped around the blanket cylinder.
The distortion caused by mounting tension can vary considerably. The average tension is estimated to be about 50 lbs. per inch of width. Distortion will depend not only on the average applied tension but on a number of other factors including the thickness of the carrier material and the diameter of the blanket cylinder. Distortion may occur in both the circumferential direction and the axial direction.
Pre-installation compensation for distortion in a raised surface spot image carrier material entails extremely detailed calculations. Software or computer programs have been developed for predicting distortion and compensating for the predicted distortion by modifying cuts which would otherwise be made in the rubber layer of a printing blanket. Nevertheless, results in distortion compensating have been inconsistent and time consuming, in part because of the various types of spot coating carrier material and the many different types of printing presses and the tension factors for proper mounting. In brief, the chance for error when calibrating for distortion is significant.